1134

THEORY AND PRACTICE OF ENGINEERING.

Book II.

that of a column of liquid which measures the atmosphericpressure. The reservoir is inclined on all sides, only com-municating with the external air by the tube em, which isopen at the two ends, and whose lower extremity, e, is in ahorizontal plane drawn through a given point/.- the re-servoir and the vase abed being full, if the orifice gh beopened, the liquid will flow through it, and sink belowthe bottom, cd , of the reservoir; at the same time theatmospheric air will enter by the tube em, and replacethe liquid : the point e of the bed, ef of the liquid supportsthe atmospheric pressure, each of the points of the samebed supporting an equal pressure, there being two variablepressures, owing one to the elastic force of the air, more orless diluted, which presses the upper surface of the liquid,the other to the height of the liquid above the same point;whence it follows that the liquid contained in the reservoir willflow through the orifice gh> under a constant pressure, cm,as long as the level of the liquid is above the bed lef

To refil the reservoir shut the orifice ^ h, open the top or stopper placed on the upperpart of the reservoir; after having filled it shut the stopcock, and again open theorifice gh.

Of the lateral Communication of Motion in Fluids when at rest. — Venturi, Professor ofNatural History at Modena in 1798, gave an account of his investigations on this subject,and was almost the first of modern philosophers who turned his attention to it. To provethat the motion of a fluid is communicated to the lateral parts which are at rest, he intro-duced the horizontal cylindrical pipe A C into thevessel D E F B, filled with water as high as D B ; op-posite, and at a small interval from the aperture C,was the commencement of a small rectangular channelof tinned iron, SMBR, open at the top, SR, theinclined bottom M B resting on the edge of the vesselB. The breadth of the channel was 24 lines; thediameter of the tube AC 14£; it was inserted in areservoir, and the water kept at a certain height;when the water of the reservoir was permitted to flowthrough A C , the current rose along the channel M B,and rushed out of the vessel, producing a current BVin the vessel D E F B ; the fluid was carried into thechannel S R, and issued at B along with the water inthe reservoir ; in a few seconds the water, D B, fell to M II.

Light bodies placed near a stream of water issuing from a reservoir exhibit a similareffect; they are impelled by the air which descends with the stream. The lateral partsof a fluid by these experiments are proved to be carried along with any stream that flowsthrough it, and consequently the motion is communicated to the lateral parts, whichare at rest. This principle Venturi applied to explain the theory of the water-blowingmachine, and also to show that the eddies in rivers are caused by the motion communicatedfrom the more rapid parts of the current to the lateral and tranquil portions : lie alsoexhibited a method of draining a piece of ground on a lower level than the current, whichhas been tried with some success, by means of a fall of water without the aid ofmachinery : when a vessel is filled w T ith a fluid, and is at rest, or in equilibrio, all theparticles of the fluid are equally pressed in every direction ; but when a small hole is madeat the bottom or in the side of the vessel, the fluid immediately over it will descend by theforce of its own gravity.

In a vessel allowed to empty itself by a hole at the bottom, the surface of the fluid pre-serves its level state until it comes to within half an inch of the bottom, when a funnel-shaped cavity appears on the surface ; when it issues from a hole in the side of the vessel,it preserves its level until lowered to the upper edge of the hole, when it inclines a little,and assumes a hollow form. The particles of a fluid move towards the hole in directionswhich converge to a point outside the orifice, so that the column of fluid whichissues from the vessel must have a smaller diameter than the orifice itself; Sir IsaacNewton , who first observed this effect, called it the vena contracta, or the contraction of thefluid vein. The distance from the orifice, where the greatest contraction takes place, isequal to half the diameter of the hole; and the area of the section of the vein at this place isto the area of the orifice, as 10 to 14’14 according to Newton, or 10 to 16 according toBossut, who also noted that with a short cylindrical tube, applied instead of a mere aper-ture, the vena contracta was to that of the orifice as 10 to 12*3.

The pressure of the atmosphere increases the expenditure of water through a simple

R

Fig. 1723.

m

Fig. 1722.